1. Field of the Invention
The present invention relates to novel processes for the continuous production of stoichiometric or non-stoichiometric electrode-quality metal oxides.
2. State of the Art
Solid-state lithium electrochemical cells are known in the art and typically consist of a lithium-containing metal anode, a lithium ion-conducting solid electrolyte and a cathode comprising a lithium ion insertion electrode material. Examples of such cathodic materials include V.sub.2 O.sub.5, TiS.sub.2, V.sub.6 O.sub.13 and LiCoO.sub.2.
Such intercalation materials are capable of acting as a cathode by virtue of their ability to reversibly accommodate lithium ions physically inserted into their crystallographic structure during discharge of the cell, and subsequently removed therefrom during charging of the cell. The amount of lithium and its ease of insertion in the cathode material depends on the crystallographic lattice structure of the material, and the number and nature of its lattice defects, as well as on the composition of the material. Therefore, materials of the same empirical formula will differ remarkably in their properties as cathode materials. For example, VO.sub.y, where y=2.5, is the empirical formula of both V.sub.2 O.sub.5 and V.sub.6 O.sub.15 ; but the former is the stable vanadium pentoxide having well known drawbacks as a cathode active material, while the latter is the limiting composition of solid vanadium oxide and has a structure similar to V.sub.6 O.sub.13.
Like other elements in the transition metal group including niobium and tantalum, vanadium forms numerous and frequently complicated compounds because of its variable valance. The four principle oxidation states of vanadium are 2+, 3+, 4+ and 5+, and it forms derivatives from more or less well defined radicals such as VO.sup.2+ and VO.sup.3+. However, vanadium oxide solids possess nominal stoichiometries which indicate a mixture of vanadium oxidation states can be present in the solid phases of vanadium.
Solid lithium electrochemical cells using V.sub.6 O.sub.13 (VO.sub.2.16) as the cathode active material are well studied. K. West et al., J. Power Sources 14 (1985) 235, studied V.sub.6 O.sub.13 as a cathode material for lithium cells using polymeric electrolytes. They found the lithium insertion reaction was reversible in the composition interval Li.sub.x V.sub.6 O.sub.13 [0.ltoreq.x.ltoreq.8]. The high stoichiometric energy density for the ultimate composition Li.sub.x V.sub.6 O.sub.13, 890 Wh/kg, is very favorable for battery applications. But this study indicated unfavorable capacity decreases occur upon cycling the Li.sub.x V.sub.6 O.sub.13 cells.
D. W. Murphy et at., J. Electrochemical Soc., 128 (1981) 2053, report the synthesis of V.sub.6 O.sub.13+x [0&lt;x.ltoreq.0.5], corresponding to the empirical formula (VO.sub.2.16 --VO.sub.2.25), by thermal decomposition of NH.sub.4 VO.sub.3 under a stream of argon. The product's powder x-ray diffraction pattern was similar to that of V.sub.6 O.sub.13. Murphy et at. studied the vanadium oxides V.sub.3 O.sub.7, V.sub.4 O.sub.9, V.sub.6 O.sub.13 and V.sub.6 O.sub.13+x [0&lt;x&lt;0.5] as cathode materials in ambient temperature non-aqueous secondary lithium cells. According to Murphy et at., the best cathode materials were V.sub.6 O.sub.13 and the slightly oxygen rich V.sub.6 O.sub.13+x [0&lt;x.ltoreq.0.2], i.e. corresponding to (VO.sub.2.16 --VO.sub.2.20). Only these cathode materials consistently exhibited substantial capacities, good rechargability, and high average potentials. Therefore, Murphy et al. concluded that these materials make the best candidates for use as cathode active materials in non-aqueous lithium secondary batteries. However, the discharge capacity of cells containing these materials as cathode active materials was found to diminish upon cycling and was found to be very temperature dependent.
U.S. Pat. No. 4,228,226 reports that vanadium oxides with nominal compositions close to V.sub.6 O.sub.13 having empirical formulas from VO.sub.2.1 --VO.sub.2.2, i.e. corresponding to V.sub.6 O.sub.13+x [0.ltoreq.x.ltoreq.0.2], are readily prepared by the thermal decomposition of NH.sub.4 VO.sub.3 at a controlled rate in an inert or reducing atmosphere at a temperature of approximately 450.degree. C. This patent also reports that vanadium oxides which have empirical formula VO.sub.2+y [0.ltoreq.y.ltoreq.0.4] are useful as cathode materials in non-aqueous cells using lithium or lithium alloy as the anode. In a preferred embodiment, it was reported that the active cathode material has nominal stoichiometry V.sub.6 O.sub.13 and the empirical formula (VO.sub.2.1 --VO.sub.2.2). However, the variation of cell capacity when using such materials as the cathode active component in a lithium cell was not reported.
U.S. Pat. No. 4,075,397 discloses, amongst many cathode active materials, a material of empirical formula VO.sub.y [1.8.ltoreq.y.ltoreq.3.2], exemplified by V.sub.2 O.sub.5, i.e. VO.sub.2.5.
It would be advantageous to create new vanadium oxides of crystallographic structure close to that of V.sub.6 O.sub.13 but having superior properties as cathode active materials.
Intercalation compounds for electrochemical cathodes are normally based on transition metal oxides produced in batch processes of limited capacity. Batch processes are difficult to scale-up. Interest is focused on ammonium metal oxides, particularly ammonium transition metal oxides which are the source of numerous intercalation compounds, such as V.sub.6 O.sub.13, that find extensive use in lithium secondary batteries.
In the production of electrode-quality metal oxides, ammonium metal oxides are heated to high temperatures to release ammonia by thermal decomposition. Ammonia decomposes at high temperature according to the equilibrium reaction NH.sub.3 .revreaction.N.sub.2 +3H.sub.2. High temperatures and low pressures push the equilibrium to the right. The presence of hydrogen can reduce metal oxides to lower oxides with the simultaneous production of water vapor. By the appropriate control of temperature and pressure, and by the addition or subtraction of inert gas and ammonia, it is possible to control the oxidation state of the metal oxides produced. For example, using ammonium metavanadate as the feed stock, thermal decomposition produces V.sub.6 O.sub.13 and higher oxides of vanadium. EQU 6NH.sub.4 VO.sub.3 .fwdarw.V.sub.6 O.sub.13 +6NH.sub.3 +3H.sub.2 O+O.sub.2 EQU 2NH.sub.4 VO.sub.3 .fwdarw.V.sub.2 O.sub.5 +2NH.sub.3 +H.sub.2 O EQU V.sub.6 O.sub.13 +O.sub.2 .fwdarw.3V.sub.2 O.sub.5 EQU V.sub.2 O.sub.5 +H.sub.2 .fwdarw.V.sub.2 O.sub.4 +H.sub.2 O
It is difficult to control the ammonia content of a batch reactor during the thermal decomposition of ammonium vanadate so as to produce V.sub.6 O.sub.13 of very high purity.
U.S. Pat. No. 4,486,400 describes a process for preparing stoichiometric V.sub.6 O.sub.13 usable as cathode active material, by means of a three-step process involving: (1) the heating of ammonium metavanadate in a dynamic atmosphere of nitrogen gas; (2) holding the vanadium oxide so produced at 350.degree.-400.degree. C. for 4 hours in nitrogen; and (3) heating the vanadium oxide at about 400.degree.-500.degree. C. for 8-12 hours in a dynamic atmosphere including oxygen at the partial pressure of oxygen over stoichiometric V.sub.6 O.sub.13 at that temperature. U.S. Pat. No. 4,619,822 describes a process for the synthesis of V.sub.6 O.sub.13 by the reduction of V.sub.2 O.sub.5 in the presence of a reducing gas which is a mixture of CO and CO.sub.2, i.e., V.sub.2 O.sub.5 +2CO.fwdarw.2CO.sub.2 +V.sub.6 O.sub.13. U.S. Pat. Nos. 4,119,707 and 3,333,916 describe processes for the production of V.sub.2 O.sub.5 from ammonium metavanadate.